impermissible

Something impermissible isn't allowed. It's impermissible in every part of the U.S. to drive 100 miles an hour on the highway.

Things that are permissible are legal, authorized, or welcome — you are permitted to do them. The opposite of permissible is impermissible, which adds the "opposite" prefix im- to permissible, from the Latin permittere, "let, allow, grant, or permit." It's impermissible to bully other kids at school, or against the rules, and it's impermissible to steal a car, or against the law.